cosmac - RCA COSMAC CDP1802 functional equivalent CPU core in VHDL

cosmac development is hosted at the cosmac Github repository.

Introduction

The RCA "COSMAC" CDP1802 microprocessor was one of the earliest CMOS microprocessors, introduced in 1976. Compared to the contemporary NMOS microprocessors, the 1802 was slower, but consumed much less power.

This project contains an implementation of a CPU core in VHDL (cosmac.vhdl) which is object-code compatible with the CDP1802. The CPU core is written in synthesizable VHDL, with no vendor-specific constructs.

The electrical interface of the core is similar to the CDP1802 but not identical. Due to the lack of true bidirectional signals in FPGAs, there are separate 8-bit data bus inputs and outputs.

The core executes each machine cycle in one clock cycle, whereas the CDP1802 required eight clock cycles per machine cycle. In a Xilinx XC7A100T-1FGG484 FPGA, the core can run with a 62.5 MHz clock, giving it performance equivalent to a 500 MHz CDP1802.

A PIXIE graphics core equivalent to the CPD1861 is also provided. The PIXIE core uses a dual-port frame buffer to allow NTSC-rate video output independent of the CPU core clock rate.

COSMAC ELF Demonstration system

A demonstration system equivalent to a COSMAC ELF microcomputer, as described in a series of Popular Electronics articles in 1976, is provided. Additional hardware-specific files are needed depending on what FPGA you are using. Suitable files for the Digilent CMOD-A7 module using the Xilinx Artix 7 FPGA are provided.

Source files

CPU core:

Filename	Description
cosmac.vhdl	CPU core

PIXIE graphics core (in "pixie" directory"):

Filename	Description
pixie_dp.vhdl	PIXIE graphics core, top level
pixie_dp_front_end.vhdl	front end (processor bus side)
pixie_dp_frame_buffer.vhdl	frame buffer
pixie_dp_back_end.vhdl	back end (composite video output side)

COSMAC ELF demo:

Filename	Description
elf.vhdl	COSMAC ELF control and interconnect
debouncer.vhdl	general-purpose switch debouncer
memory.vhdl	64KB static RAM

FPGA-specific source files for COSMAC ELF demo:

Directory	Description
elf-cmod-a7	Digilent CMOD-A7 module using Xilinx Artix-7
elf-x7	(deprecated) Xilinx 7-Series FPGAs (e.g. XC7A100T-1FGG484)

Status

The following features of the core have been tested and appear to work correctly:

Most of the instruction set has been tested by running CamelForth.
The DMA input and "load mode" capability have been tested by using the core as part of a COSMAC ELF equivalent.

The following features have not been tested:

The DMA output feature of the CPU core had had only minimal testing.
The interrupt feature of the CPU core and the related instructions have had only minimal testing.

License information

For licensing purposes, the VHDL source files both individually and collectively are considered a "program", licensed under the terms of the GNU General Public License 3.0, which is provided in the file gpl-3.0.txt. If you distribute this program as part of a derived work (including but not limited to use in an FPGA or ASIC), the license imposes obligations on you to make the source code for the entire work available. No one forces you to accept this license, but if you choose not to accept it, nothing authorizes you to redistribute this program in any form.